Summary of Work: A major focus of this project is to discover the roles of Amyloid Precursor Protein (APP) and Presenilins (PS) in the etiology and pathology of Alzheimer?s Disease (AD). The normal physiological roles of these proteins are also under investigation. These proteins are important to study since the processing of APP and the effect of PS on APP processing bear directly on the increased production and deposition of A beta peptides in senile plaques of AD. The processing of APP also generates secreted forms of the protein which may have neurotrophic properties. Brains of AD patients exhibit selective and massive neuronal loss. In light of recent evidence from our laboratory showing that, in cell culture, over-expression of familial AD mutated forms of APP and PS causes cell death, we are interested in examining the mechanisms involved in this cell death. One of the aims of our laboratory is to discover how APP or PS mutations lead to specific neuronal cell loss in AD. Previously we showed that over-expression of mutated forms of APP in stably transfected PC12 cells led to an increased production of intracellular, amyloidogenic C-terminal fragments of APP. This was accompanied by increased apoptotic cell death over several days. We now have shown that this death is prevented by low concentrations of vitamin E suggesting increased oxidative stress as a mechanism leading to cell death. We also showed that over-expression of FAD mutant APP by adenovirus-mediated gene transfer leads to cell death of primary hippocampal neurons. Interestingly, over-expression of antisense APP also causes increased cell death in these cells suggesting a neurotrophic function for endogenous APP. This conclusion was substantiated by results showing that treatment of PC12 cells with a truncated secreted form of APP lacking the entire A beta sequence at its carboxyl terminus did not have neurotrophic activity. However, normal secreted APP supported the differentiation of PC12 cells and potentiated NGF induced differentiation. In collaborative studies, we have shown that neurons lacking PS1 fail to produce A beta and release more secreted neurotrophic forms of APP. These results suggest that in AD, the selective neuronal cell loss may be, in part, due to an oxidative stress induced apoptotic mechanism. This cell death in AD may result from an imbalance in the generation of toxic and neurotrophic processing products of APP. This provides a rationale for targeting particular elements of apoptotic pathways and APP processing for therapeutic intervention in AD.